The present invention relates to pressure regulating valves and more particularly to pilot operated pressure regulating valves which include a pilot mechanism for supplying a loading fluid to one side of a main valve pressure responsive element which controls the flow rate through the valve.
Prior art pilot operated pressure regulating or reducing valves generally utilize a pilot mechanism which senses the pressure of the fluid to be controlled (normally the pressure downstream of the main valve) and varies the pressure on the main valve pressure responsive element to open and close the main valve to control the flow rate through the valve, and thus the pressure drop across the valve to thereby control pressure of the fluid downstream of the valve.
In typical prior art pilot operated pressure reducing valves, the pressure sensing mechanism of the pilot mechanism comprises a diaphragm or other pressure responsive element which has the downstream pressure applied to one side and an adjustable force applying member applied to the other side, such as for example an adjusting spring mechanism. Changes in downstream pressure cause motion of the pilot pressure responsive element which is then transmitted to a small pilot valve member which opens and closes to vary the supply of a relatively high loading pressure to the main valve pressure responsive element. More particularly, the high pressure loading fluid may for example be taken from the upstream side of the main valve assembly, and the outlet of the pilot valve may be connected to the area above the main valve pressure responsive element. Thus, as the downstream pressure drops below the set point (adjusted by the adjusting spring) the pilot pressure responsive element will move in a direction to open the pilot valve to thus increase the loading pressure to the main valve diaphragm which in turn opens the main valve to increase the flow therethrough and thus the outlet pressure.
For an increase in the outlet pressure of the main valve, an opposite action occurs to decrease the main valve pressure responsive element loading pressure to in turn reduce the main valve opening. More particularly, an increase in the downstream pressure causes the pilot valve sensing diaphragm to move in a direction to close the pilot valve. A small fixed bleed orifice or other bleed leakage mechanism is connected between the outlet side of the pilot valve and the low pressure downstream side of the main valve in order to remove loading pressure from the main valve diaphragm. This small fixed bleed orifice is always open, even during normal steady operation to provide a continuous bleed leakage. Thus, during normal operation, the pilot valve is usually unseated from its valve seat so that loading pressure is always being supplied to the area above the main valve diaphragm, the rate of flow of loading fluid supplied substantially corresponding to the flow rate through the bleed orifice.
One problem encountered with prior art pilot operated pressure regulating valves has been the gain or sensitivity of the pilot mechanism which is affected by the system pressure drop (i.e., the inlet pressure minus the outlet pressure). For high pressure drops, very small motions of the pilot valve cause large changes in the diaphragm or loading pressure thus providing the high gain. At low pressure drops on the other hand, the opposite occurs. If the valve is designed with some minimum acceptable gain for low pressure drop conditions, it tends to be unstable at high pressure drops. That is, when designed to provide an acceptable gain for low pressure drop conditions, the mechanism tends to overcompensate or overshoot the desired limit at high pressure drops and thus unbalance the outlet pressure in the opposite direction. Such action could continue for a considerable period of time before the regulator is stabilized since the pressure impulse caused by the repositioning of the main valve is not transmitted immediately to the pilot diaphragm.
One prior art arrangement aimed at overcoming this "hunting" effect is U.S. Pat. No. 2,277,162 to Soderberg which utilizes double diaphragms or pressure responsive elements for pilot operated regulators. The double diaphragms are operatively connected together with the pressure from the downstream side of the main valve (the fluid to be controlled) introduced therebetween. A relatively high pressure loading fluid is subjected to the lower side of the lowermost diaphragm (against the downstream pressure acting on the other side). On the upper side of the upper diaphragm, a spring acts against the force of the downstream pressure acting on the upper diaphragm. The arrangement is such that the effective pressure area of the lower diaphragm on which the downstream pressure acts is progressively decreases as it moves upwardly (i.e., as the pilot valve closes further in response to increases in downstream pressure), whereas the effective pressure area is progressively increased when the diaphragm moves downwardly (i.e., as the pilot valve opens further in response to decreases in downstream pressure). This change in effective pressure area serves to counteract the downstream pressure changes to prevent overregulation or "hunting" so that a balanced condition will be achieved more quickly, and movement of the main regulating valve reduced.
However, with the Soderberg arrangement, as with the other prior art arrangements, a continuous leakage of the loading pressure is provided to release excess pressure or fluid introduced onto the main pressure responsive elements. Thus, when the downstream pressure increases beyond the preset limit the arrangement is such that the pilot valve supplying the loading pressure to the main valve diaphragm tends to close with the excess loading fluid being removed from the main pressure responsive element only through the bleed leakage path. In the Soderberg reference, this leakage path is arranged between the main valve pressure responsive piston and the side of the cylinder within which it moves. As can be appreciated, for large changes in downstream pressure, it may take a relatively considerable amount of time for the excess loading pressure to be directed away from the main pressure responsive element, thus slowing the response time for the valve. On the other hand, if a large continuous bleed opening were included to provide for a quicker response for the main valve, small changes in pressure drop and fluid turbulence can lead to instability or "chatter" of the main valve.